Tire sensor

ABSTRACT

The present invention relates to a tire sensor, especially tire sidewall torsion sensor (=SWT sensor), including at least two pick-ups for measuring data being mounted at a distance from the tire rotational axis on the chassis, the said pick-ups for measuring data interacting with at least one encoder mounted on or in the tire wall or with at least one conventional encoder mounted on or in the tire wall and exhibiting poles, wherein the output signals or output information of such pick-ups for measuring data sensors are/is transmitted to the motor vehicle control system. 
     In order to send the signals provided by the tire sensor to an evaluation unit or the motor vehicle control system in a fashion rid of errors that are due to influence quantities, at least one analog and at least one digital signal conditioning and/or processing unit is provided between the motor vehicle control system and the pick-ups for measuring data, the pick-ups for measuring data send at least two output signals that can be evaluated with respect to a change in the phase position and/or at least one output signal that can be evaluated with respect to the change of the amplitude to the digital signal-conditioning and/processing unit, and the digital signal-conditioning and/or processing unit calibrates the systematic errors of the output signals with respect to whether the error is an amplitude-related error or a phase-related error.

The present invention relates to a tire sensor, especially tire sidewalltorsion sensor=SWT sensor, including at least two pick-ups for measuringdata being mounted at a distance from the tire rotational axis on thechassis, the said pick-ups for measuring data interacting with at leastone encoder mounted on or in the tire wall or with at least oneconventional encoder mounted on or in the tire wall and exhibitingpoles, wherein the output signals or output information of such pick-upsfor measuring data sensors are/is transmitted to the motor vehiclecontrol system.

Many methods and devices using tire sensors for detecting the forces andtorque acting on the tires are known for controlling the drivingperformance of a motor vehicle. Tire sensors (SWT sensors) consist ofone encoder mounted in or on the tire and at least one pick-up formeasuring data that is mounted on the chassis in a stationary manner andassociated with the encoder. Whereas EP 04 441 09 B1 proposes monitoringthe deformation of the range of the tire profile, i.e., the tire contactarea, WO 96/10505 proposes detecting the deformation of the sidewall ofa tire—the torsion deformation—by measuring a period of time thatelapses between the passing of at least two markings arranged on therotating wheel at a different radius in relation to the rotational axis.The longitudinal forces that act on the tire are inferred from theevaluation of the signals obtained as described above. In addition, thetransverse forces may be determined from variations of the amplitudes ofthe signal sensed by the pick-up for measuring data which representschanges in the distance between the pick-up for measuring data and theencoder. A tire sensor which detects a change in the phase positionbetween output signals emitted by pick-ups for measuring data when thetire is deformed due to forces acting on the tire is described in WO97/44673.

Further, a method is disclosed in WO 99/19192 which, on the basis of theforces that act on the individual wheels and are sensed by tire sensors,determines condition variables of the vehicle which satisfy the highdemands placed on motor vehicle control systems in terms of accuracy andreliability.

An object of the present invention is to send the signals provided bythe tire sensor to an evaluation unit or the motor vehicle controlsystem rid of errors that are due to influence quantities.

According to the present invention, this object is achieved by a generictire sensor with the special characteristics that at least one analogand at least one digital signal conditioning and/or processing unit areprovided between the motor vehicle control system and the pick-ups formeasuring data, that the pick-ups for measuring data send at least twooutput signals that can be evaluated with respect to a change in thephase position and/or at least one output signal that can be evaluatedwith respect to the change of the amplitude to the digitalsignal-conditioning and/processing unit, and that the digitalsignal-conditioning and/or processing unit calibrates the systematicerrors of the output signals with respect to whether the error is anamplitude-related error or a phase-related error.

The tire sensor of the present invention and a control based thereon,hence, founds on the forces that occur directly at the tire. Thispermits detecting all influence quantities and wrong interpretationswhich are due to ambiguous signals or errors in processing what impairsthe determination of vehicle condition variables or quantitiesdescribing the vehicle behavior. An error quantity which is the basis ofthe respective electric characteristic quantities and is caused bydifferent influence quantities is taken into consideration by thecalibration of the generally sinusoidal signals, provided by thepick-up(s) for measuring data with respect to the signals' electriccharacteristic quantity. The envisaged signal-conditioning and/orprocessing unit reduces the quantity of data of several interlinkedanalog input signals in a favorable manner so that further processing ofthe information obtained is ensured with reduced effort and structure inthe digital signal processing. In this arrangement, the changes of theamplitude, the period duration, and the phase relation of the inputsignals permits obtaining the information which render it possible tocalculate the transverse and longitudinal forces that act on the tires.The signal-conditioning and/or processing unit comprises a means todetect undesirable error quantities and to compensate them numericallyin the subsequent data processing operation.

In a favorable embodiment of the present invention, theamplitude-related error is compensated by a multiplicative calibrationand the phase-related error is compensated by an additive calibration inthe digital signal-conditioning and/or processing unit. According to thepresent invention, the amplitude-related output signal is rectified andthe maximum values (amplitudes) determined in the analogsignal-conditioning unit, while in the digital signal-conditioningand/or processing unit the offset is determined by way of averaging themaximum values with correct signs and a multiplier f is formed of themaximum value divided by the mean value of the rectified signal andassigned to each output signal or each pole of the encoder. Associatedwith the phase-related output signal of the tire sensor is, according tothe present invention, an ideal pole pattern which corresponds to thenumber of poles and preferably has equidistant poles. In the digitalsignal-conditioning and/or processing unit, the ideal pole pattern iscompared to each pole of the encoder of the phase-related output signal.In dependence on the comparison, correction factors are then formed orupdated which are associated additively with the output signal accordingto the present invention.

Thus, the present invention is based on the knowledge that thecorrection of the amplitude error can be based on; the assumption thatthe error is a multiplicative error because additive air slot variations(error: Δ) or tire sidewall equalities can be presupposed which, by wayof the exponential characteristic curve

|Amplitude−Offset|=A*exp(B*air slot) become a multiplicative amplitudevariation:

|Amplitude−Offset|=A*exp(B*(air slot+Δ)

|Amplitude−Offset|=A*exp(B*air slot+Δ′)

|Amplitude−Offset|=A*exp(B*air slot)* exp(Δ′)

|Amplitude−Offset|=A*exp(B*air slot)* Δ″

and a multiplicative inequality of the magnetic field is presupposedwhich is directly proportional to the amplitude and thereby generates amultiplicative inequality of the amplitude.

The correction of the pole division error, however, is based on theassumption that the pole division error is an additive error because itis an angle error which changes additively over the periphery of theencoder 17.

Favorably, the amplitudes and phase differences are stored in a ringmemory for error correction, corresponding to the pole number, such as1, 2, . . . to 96.

The calibration of the present invention permits immediately correctingerrors that occur periodically on a wheel, without the need foreffecting a time-consuming (delay time) filtering operation. It wasfound out in tests that the accuracy of signal evaluation can beenhanced by up to 7 percent due to the calibration.

An embodiment of the present invention will be described in detailhereinbelow by making reference to the accompanying drawings.

In the drawings,

FIG. 1 is a schematic wiring diagram of the SWT-signal-conditioning andprocessing unit.

FIG. 2 is a schematic representation of the causes of the amplitudeerror.

FIG. 3 shows in a characteristic curve the dependence of the amount ofthe output signal amplitude on the air slot.

FIG. 4 is a representation of the amplitude error by an offset (d-ccomponent) and by irregularity.

FIG. 5 is a representation of the error in the phase difference.

FIG. 6 shows the signal flow between the signal-conditioning andprocessing and the motor vehicle control system.

FIG. 7 shows the principle of the compensation of offset andirregularity errors.

The circuit illustrated in the FIG. 1 embodiment comprises the followingfunction groups:

analog signal conditioning 14

digital signal conditioning 15 a

digital signal or data processing 15 b

(DSP=digital signal processor)

CAN interface providing a connection to the motor vehicle control system13.

In the analog signal conditioning, the sinusoidal output current signalsof the SWT pick-ups for measuring data 10, 11 are transferred into avoltage and converted into a squarewave signal, filtered, adapted tochanges of the signal offset of the sensors, and the peak value of everyhalf wave is detected.

In the digital signal conditioning 15 a, the analog signals areconverted to digital signals with respect to the amplitude, the period,and the time delay.

In the digital signal or data processing 15 b, the wheel speed and wheelforces are determined. In addition, pole division errors and amplitudeerrors are compensated.

The amplitude errors are due to an irregularity of the tires in alateral direction. FIG. 2 shows the tire sidewall irregularityschematically as an undulated line 16. The tire sidewall irregularitycauses variations of the air slot between the pick-ups for measuringdata (sensors) 10 and, respectively, 11 which are attached stationarilyon the chassis at a distance from the axis of rotation of the wheel orthe tire, and the encoder 17 arranged in the tire sidewall (FIG. 5),which irregularity does not occur under the effect of forces acting onthe tire. Rather, there is a periodic change in distance on a wheelwhich correlates with the amplitude detected by the analog signalconditioning 14. The correlation between the amplitude and the air slotor distance between the pick-ups for measuring data 10, 11 and theencoder 17 is depicted in FIG. 3, the air slot being plotted inmillimeters (mm) on the abscissa and the amplitude being plotted inMilli volt (mV) on the ordinate, rid of a d-c component (offset).Another amplitude error is based on the irregularity of; themagnetization of the tire sidewall over the periphery of the sidewall sothat an additional wheel periodic variation of the amplitude is produceddue to the irregularity of the magnetic field or the change in themagnetic field strength indicated in FIG. 2. Further, an amplitude errormay develop from the fact that the output signal comprises an a-ccomponent and a d-c component 19 (offset) which is filtered out in theanalog signal conditioning 14. The amplitude is obtained from theresidual a-c component by rectification with a subsequent determinationof the maximum value. It is possible to determine the offsetstationarily to an insufficient degree or with time delay. In that case,the maximum values of the half waves (amplitudes) are different due tothe irregularity component 18 (FIG. 4).

Phase errors are due to a deviation of the applied magnetization fromthe ideal pole pattern, which is based on a uniform (equidistant)distribution of the poles of the encoder 17 over the periphery of thetire sidewall. This applies to assemblies which include one tire sensorand one conventional sensor or two tire sensors with two pick-ups formeasuring data 10, 11.

The errors of the output signal of the pick-ups for measuring data 10,11 are corrected as follows:

The calibration or initialization of the error correction is effectedwhen the driving behavior is stationary. The stationary driving behavioris determined by means of input quantities which are furnished byconventional sensors and e.g. comprise the transverse and longitudinalacceleration variation and the yaw rate acceleration. Suitably, no oronly very small force variations act on the wheel or the tire at thistime. The following conditions can be made the basis for a stationarydriving behavior:

|transverse and longitudinal acceleration variation|<0,05 g/s

|yaw rate acceleration|<5 degrees/s²

When the conditions are satisfied, the calibration of the output signalmay be performed.

In a ring memory of the digital signal conditioning 15 a, the amplitudesA₁, A₂ and the phase difference Δφ during a stationary driving behaviorare memorized according to the following table:

Pole Number 1 2 3 . . 96 A₁ A₁ (1) A₁ (2) A₁ (3) . . A₁ (96) A₂ A₂ (1)A₂ (2) A₂ (3) . . A₂ (96) Δφ Δφ (1) Δφ (2) Δφ (3) . . Δφ (96)

In the preferred design, the encoder 17 has 48 pole pairs, i.e., 96poles. In the pole memory, the pole-related error is assigned to therespective pole of encoder 17. This assignment of the pole-related errorto the associated pole is maintained in the event of a change ofdirection of the vehicle by way of a synchronization of the correctionpattern to the current;pole pattern. Encoders 17 with a different polenumber may of course be used corresponding to the embodiment.

The difference in phases between the bottom pick-up for measuring data10 or a conventional sensor arranged closer to the axis of rotation ofthe wheel or the tire and the top pick-up for measuring data 11 arrangedmore remote from the axis of rotation of the wheel or tire is consideredas phase difference. To compensate the phase error, the mean value ofthe phase differences over one wheel rotation is produced according tothe following relation:${\Delta \quad \phi_{mean}} = {{\frac{\sum{\Delta \quad {\phi (i)}}}{96}\quad i} = {1\quad \ldots \quad 96}}$

To this end, the current ring memory in copied in a second ring memorywith the designation Δφ₀: second ring memory

Δφ₀ Δφ₀ (1) Δφ₀ (2) Δφ₀ (3) . . Δφ₀ (96) ↓

The second ring memory is related to the mean value: second ring memory

Δφ_(A) Δφ₀ (1) − Δφ₀ (2) − Δφ₀ (3) − . . Δφ₀ (96) − Δφ_(mean) Δφ_(mean)Δφ_(mean) Δφ_(mean)

The additive phase error is then contained in the second ring memoryaccording to the following relation or replaces the ring memory Δφ₀which then contains the mean value of the phase difference during awheel rotation:

Δφ_(A)=Δφ₀−Δφ_(mean)

wherein Δφ_(A)=additive phase error, Δφ₀=values in the second ringmemory.

In the following correction cycles, the phase error is corrected by amodification of the phase differences (Δφ*) which are used for thefurther calculations according to the following relation:

Δφ*(i)=Δφ(i)−Δφ_(A)(i)=Δφ(i)−Δφ₀(i)+Δφ_(mean) i=1 . . . 96

The correction of the amplitude error in the presence of a stationarydriving behavior is based on the condition that the sign in theamplitude information is taken into account and that uneven pole numberspertain to the positive half wave (A(2*i+1)>0, i=0 . . . 48) and theeven pole numbers pertain to the negative half waves (A(2*i)<0, i=0 . .. 48). The compensation of the amplitude error is effected in threesteps:

1. detecting the error of the d-c component (offset)

2. detecting the error of the irregularity component

3. compensation of the error(s)

The compensation of the amplitude error of the amplitude A₁ will bedescribed in the following. Amplitude A₂ shall be treatedcorrespondingly.

The current ring memory A₁ is copied into a second ring memory with thedesignation A₍₀₎:

second ring memory

A₍₀₎₁ A₍₀₎₁ (1) A₍₀₎₁ (2) A₍₀₎₁ (3) . . A₍₀₎₁ (96)

Because the amplitude contains the correct sign, the d-c component of aperiod can be found by summing two subsequent amplitudes A(2*i+1) andA(2*i+2) or the difference of two subsequent amplitudes |A(2*i+1)|and|A(2*i+2|. The d-c component A_(offset) over the total tireperiphery may then be determined according to the following relation:$A_{Offset} = {{\frac{\sum{A_{0}(i)}}{96}\quad i} = {1\quad \ldots \quad 96}}$

The second ring memory A₀ is now replaced by the ring memory a₀ whichcontains the amount of the amplitude compensated by the offset accordingto the following relation:

a ₀(i)=|A ₀(i)−A _(offset) |i=1 . . . 96

second ring memory

a₍₀₎ ₁ | a₍₀₎ ₁ (1) − | a₍₀₎ ₁ (2) − | a₍₀₎ ₁ (3) − . . | a₍₀₎ ₁ (96) −A_(offset) | A_(offset) | A_(Offset) | A_(Offset) |

A multiplicative change of the calculation is made the basis whendetermining the amplitude error due to the irregularity of the tiresidewall 16 and the magnetization (different field strength) of theencoder 17. The mean value of the irregularity over a wheel rotation iscalculated according to the following relation:$a_{mean} = {{\frac{\sum{a_{0}(i)}}{96}\quad i} = {1\quad \ldots \quad 96}}$

The multiplicative error is calculated from the difference between theamount compensated by the offset and the mean value according to thefollowing relation illustrated in FIG. 7.${f(i)} = {\frac{a_{0}}{a_{mean}} = {{\frac{{{A_{0}(i)} - A_{Offset}}}{a_{mean}}\quad i} = {1\quad \ldots \quad 96}}}$

The second ring memory a₀ is replaced by the multiplicative error f(i)or, respectively, the multiplicative error is copied into the secondring memory.

f f₁ (1) f₁ (2) f₁ (3) . . f₁ (96)

As shown in the representation in FIG. 7, the amplitudes A(i) thatinclude errors are then replaced by the amount of the amplitudes A*(i)rid of the offset error and irregularity error according to thefollowing relation:${A^{*}(i)} = {{\frac{{{A(i)} - A_{Offset}}}{f(i)}\quad i} = {1\quad \ldots \quad 96}}$

In FIG. 7, right-hand diagram, the error-compensated output signal isplotted on the ordinate, and the angular frequency is plotted againsttime on the abscissa. The offset-corrected output signals are designatedby reference numeral 22 and the signals rid of offset and irregularityerrors have been assigned reference numeral 23.

A long-time filtering of the correction terms is provided according to afavorable embodiment. Interferences and influences, if any, areminimized by filtering with a low-pass filter. The correction with adiscreet low-pass filter is carried out according to the followingrelation:

f(k)=1/(1+FC)*(FC*f(k−1)+u(k), wherein

f(k)=filtered value of f at the point of time t_(k),

f(k−1)=filtered value of f at the time t_(k−1),

u(k)=value of f at the time t_(k) and

FC=filter constant (FC>0).

According to another embodiment, the multiplicative error term ischanged so that the amplitude is standardized. The mean value a_(mean)is then set to equal 1 so that${f(i)} = {\frac{a_{0}}{1} = {{{{{A_{0}(i)} - A_{Offset}}}\quad i} = {1\quad \ldots \quad 96}}}$

applies.

Correspondingly, a standardization of the phase difference may favorablybe achieved in that the mean value is set to Δφ_(mean)=0 so that

Δφ_(A)=Δφ₀−Δφ_(mean)Δφ_(mean)=0

Δφ_(A)=Δφ₀

applies.

The amplitude and the phase difference are preferably standardized whenthe driving behavior is stationary under the following conditions:

|transverse acceleration|<0.07 g

|longitudinal acceleration|<0.1 g

|steering angle|<1°

steering angle velocity<20[degree/s]

forward driving

gearshift-dependent speed

first gear  <10 km/h second gear  <30 km/h third gear  <50 km/h fourthgear <100 km/h fifth gear <150 km/h

When these conditions are satisfied and remain stable for roughly 70msec, a stationary driving behavior free from longitudinal or transverseforces prevails.

What is claimed is:
 1. Tire sensor system, comprising: at least twopick-ups for measuring data from a rotating tire, a motor vehiclecontrol system, wherein said measured data includes at least one analogsignal and at least one digital signal, a conditioning and processingunit coupled between the motor vehicle control system and the pick-upsfor conditioning and processing the measured data, wherein at least oneof the signals contains tire related data in its phase and at least oneof the signals contains tire related data in its amplitude, and whereinthe conditioning and processing unit couples output information to saidmotor vehicle control system and calibrates the systematic errors of theoutput information with respect to whether an error in the measured datais an amplitude-related error or a phase-related error, wherein theamplitude-related error is compensated by a multiplicative calibrationand the phase-related error is compensated by an additive calibration inthe conditioning and processing unit.
 2. Tire sensor as claimed in claim1, wherein the conditioning and processing unit includes an analogsignal-conditioning unit and wherein said analog signal is generallysinusoidal.
 3. Tire sensor as claimed in claim 1, wherein theconditioning and processing unit includes an analog signal-conditioningunit, and wherein the amplitude-related output signal is rectified inthe analog signal-conditioning unit and a maximum value is determined,and wherein in the conditioning or processing unit includes a digitalsignal conditioning unit, which calculates an offset by way of averagingthe maximum values with correct signs, and wherein a multiplier f isformed of the maximum value divided by the mean value of the rectifiedsignal.
 4. Tire sensor as claimed in claim 3, wherein the tire includesa number of poles, and wherein the number of poles is associated withthe phase-related output signal of the tire sensor, wherein the digitalsignal-conditioning and processing unit compares the phase-relatedoutput signal to an ideal pole pattern, and wherein, in dependence onthe comparison, correction factors are formed or updated which areassociated additively with the output signal.
 5. Tire sensor as claimedin claim 4, wherein the ideal pole pattern has equidistant poles. 6.Tire sensor as claimed in claim 4, wherein the correction factors arestored in a memory of the digital signal-conditioning or processingunit.
 7. Tire sensor as claimed in claim 1, wherein the calibration ofthe amplitude-related errors or phase-related errors is effected duringstationary driving behavior.
 8. Tire sensor system, comprising: at leasttwo pick-ups for measuring data from a rotating tire, a motor vehiclecontrol system, wherein said measured data includes at least one analogsignal and at least one digital signal, a conditioning and processingunit coupled between the motor vehicle control system and the pick-upsfor conditioning and processing the measured data, wherein at least oneof the signals contains tire related data in its phase and at least oneof the signals contains tire related data in its amplitude, and whereinthe conditioning and processing unit couples output information to saidmotor vehicle control system and calibrates the systematic errors of theoutput information with respect to whether an error in the measured datais an amplitude-related error or a phase-related error, wherein theconditioning and processing unit includes an analog signal-conditioningunit and wherein said analog signal is generally sinusoidal.
 9. Tiresensor system, comprising: at least two pick-ups for measuring data froma rotating tire, a motor vehicle control system, wherein said measureddata includes at least one analog signal and at least one digitalsignal, a conditioning and processing unit coupled between the motorvehicle control system and the pick-ups for conditioning and processingthe measured data, wherein at least one of the signals contains tirerelated data in its phase and at least one of the signals contains tirerelated data in its amplitude, and wherein the conditioning andprocessing unit couples output information to said motor vehicle controlsystem and calibrates the systematic errors of the output informationwith respect to whether an error in the measured data is anamplitude-related error or a phase-related error, wherein theconditioning and processing unit includes an analog signal-conditioningunit, and wherein the amplitude-related output signal is rectified inthe analog signal-conditioning unit and a maximum value is determined,and wherein in the conditioning or processing unit includes a digitalsignal conditioning unit, which calculates an offset by way of averagingthe maximum values with correct signs, and wherein a multiplier f isformed of the maximum value divided by the mean value of the rectifiedsignal.
 10. Tire sensor as claimed in claim 9, wherein the tire includesa number of poles, and wherein the number of poles is associated withthe phase-related output signal of the tire sensor, wherein the digitalsignal-conditioning and processing unit compares the phase-relatedoutput signal to an ideal pole pattern, and wherein, in dependence onthe comparison, correction factors are formed or updated which areassociated additively with the output signal.
 11. Tire sensor as claimedin claim 10, wherein the ideal pole pattern has equidistant poles. 12.Tire sensor as claimed in clam 10, wherein the correction factors arestored in a memory of the digital signal-conditioning or processingunit.
 13. Tire sensor system, comprising: at least two pick-ups formeasuring data from a rotating tire, a motor vehicle control system,wherein said measured data includes at least one analog signal and atleast one digital signal, a conditioning and processing unit coupledbetween the motor vehicle control system and the pick-ups forconditioning and processing the measured data, wherein at least one ofthe signals contains tire related data in its phase and at least one ofthe signals contains tire related data in its amplitude, and wherein theconditioning and processing unit couples output information to saidmotor vehicle control system and calibrates the systematic errors of theoutput information with respect to whether an error in the measured datais amplitude-related error or a phase-related error, wherein thecalibration of the amplitude-related errors or phase-related errors iseffected during stationary driving behavior.